SiC single crystals are, in particular, grown for use in different types of semiconductor devices, as for example different types of diodes, transistors and thyristors, which are intended for applications in which it is possible to benefit from the superior properties of SiC in comparison with, especially, Si, namely the capability of SiC to function well under extreme conditions. The large band gap between the valence band and the conduction band of SiC makes devices fabricated from the material able to operate at high temperatures, namely up to 1000.degree. K.
There are different techniques known for the epitaxial growth of Silicon Carbide. The technique of seeded sublimation growth is at present commonly used for growing Silicon Carbide crystals for subsequent substrate production. This technique is limited both with respect to crystalline quality and purity.
The substrates produced by this method are perforated with holes called "micropipes" and have additionally a mosaic structure related to grains of slightly different crystal orientation.
The growth of the crystals is made by subliming a source powder of SiC in a container. The SiC vapors are transported to the seed crystal by an artificially applied thermal gradient. The growth rate is determined by the degree of supersaturation of the vapors in the atmosphere around the seed crystal, which in turn is determined by the temperature, the applied temperature gradient and the pressure in the system. The vapor transport is thus characterized by diffusion processes and convection. Low pressures in the container are needed for making the transport of the sublimed SiC powder effective, while avoiding too many collisions of the vapor SiC on its way to the seed crystal. The obtained growth rates with such a system is in the order of a few mm/h. Typical temperatures, temperature gradients and pressures are in the order of 2400.degree. C. for the source material, 10-30.degree. C./cm and 5-50 millibar, respectively. The ambient is normally Ar. The advantage with this method is its simplicity. The disadvantage with the method is the limited control of the system, the unsatisfactory crystalline quality, and the low purity which largely is governed by the purity of the source material and which indeed may be improved by the choice of a purer source material. Due to an inevitable escape of Si from the quasi-closed container, the C/Si ratio of the vaporised source material cannot be kept constant during the entire growth. This affects the growth in a negative way and cause crystalline defects. In order to grow crystals of significant size for subsequent substrate production, the growth must also be interrupted from time to time to refill the container with new source material. These interruptions also disturb the growing crystal. During growth, the presence of the temperature gradient at the growth interface causes formation of crystalline defects such as micropipes, dislocation and point defect agglomerates.
Another technique used for the epitaxial growth of Silicon Carbide layers is the Chemical Vapor Deposition technique, which in terms of purity and crystalline quality is far superior to that of the seeded sublimation growth. The gas needed for the growth is transported to the substrate by a carrier gas, normally hydrogen. The precursor gases used are, in the SiC case, normally silane and propane. The precursor gases decompose or are cracked, and the silicon and carbon consituents migrate on the growing crystal surface to find a proper lattice site. The temperature of the system is normally kept below 1600.degree. C. Essentially no temperature gradient is present in the growth front of the crystal.
The advantage with the CVD process is the purity and the crystalline quality which mainly is limited by the substrate quality. The disadvantage with the CVD technique is the low growth rates which rules out any possibility of growing crystals for substrate production by this technique, or even thick high quality layers at a commercially interesting capacity. The typical growth rates of CVD grown SiC epitaxial layers are in the order of several gm/h at 1600.degree. C.
Recently another process, namely the High Temperature Chemical Vapor Deposition (HTCVD) process has been presented (paper of High Temperature Chemical Vapor Deposition released on Technical Digest of Int'l Conf. on SiC and Related Materials -ICSCRM-95-, Kyoto, Japan, 1995 and the US patent application Ser. No. 08/511 324). This process is technically a CVD process carried out at very high temperature where sublimation and etching of the seed crystal (substrate) and growing crystal or layer is significant.
The etching of the growing surface has been shown to improve the crystalline quality. Also due to the purity of the precursor gases the purity of the grown crystals is very high. The growth rate can be increased to the order of a few mm/h due to the increased surface mobility of the atoms which thereby find their correct lattice sites faster. In the HTCVD process the temperatures are in the order of 1900.degree. C.-2500.degree. C. used.
The advantage with the HTCVD process is the high purity, the high crystalline quality and also the high growth rate. The disadvantage with the technique is the difficulty to establish favorable conditions for growth in an artificial way by adding silicon and carbon precursor gases in the correct amount at all times, is during the temperature increase, to the growth temperature and during growth. If too small an amount of precursor gases is added, a too high degree of etching or sublimation may occur, which thereby may cause a graphitization of the crystal surface which will, in turn, cause crystalline defects or even totally prevent growth. If too high an amount of precursor is added, the supersaturation may be too high for the surface mobility and the growth may be polycrystalline.
The conditions must thus artificially be kept close to a thermodynamical equilibrum, which may be very delicate to achieve. The solution to this problem is given in a copending patent application filed by the applicant the same day as the present application. This technique has been considered to form the state of the art while drafting the preambles of the appended independent claims, although the inventional technique defined below is, in fact, no type of CVD technique.
Another problem is the transport of source material for the growth to the susceptor, which in the HTCVD-process is carried out for growing SiC is the transport of silane. The silane may decompose at an early stage and result in a total or partial plugging of the gas tube leading to the susceptor or a complete depletion of the gas which will disable growth. Furthermore, there is a risk of explosions when silane is used in high concentrations.